The present invention relates to an apparatus for measuring the content or concentration of hydrogen dissolved in a molten metal. More particularly, the invention is concerned with improvements in an apparatus for direct measurement of the dissolved hydrogen-content of liquid metals, more specifically, of molten aluminum or aluminum alloys.
It was known that a hydrogen dissolved in molten aluminum, aluminum alloy or other molten metals causes voids or pores to develop within a body or batch of such molten metals during a process of solidification thereof, which pores give rise to various sorts of drawbacks or defects of the end products. Although these defects due to the pores had not been considered a serious problem, there has been an increasing requirement in recent years that so-called "dehydrogenation" be performed, i.e., the hydrogen dissolved in a molten metal be removed, as an important step of a metal casting process. Such demand has been increased keeping pace with an increasing demand for high-quality products, for example, structural aluminum panelings with fine surface finish as with anodic oxidation. In the anodic oxidation of such panelings, developments of minute "blisters" on the surfaces of aluminum sheets during a hot-processing operation will induce defects in the form of pits on the surface-finished panelings thereby degrading the quality of the end products.
In general, the dehydrogenation step requires a considerable period of time but the dehydrogenation, if conducted for a period longer than required, will lead to an elevated cost of manufacture of the end products. In consideration of these facts, it has been deemed to be a common practice and recommended to measure the content or concentration of hydrogen dissolved in a molten metal and use the measurements to keep the length of dehydrogenation to a necessary minumum.
In light of the above requirement for minimizing the dehydrogenation time and the consequent requirement for reducing the time spent in measuring the hydrogen-content of the molten metal, it has been proposed to adopt the so-called "Telegas" process wherein the measurement is carried out by an apparatus as shown in the U.K. Pat. No. 684,865 and the U.S. Pat. No. 2,861,450, the disclosure of which is hereby incorporated by reference.
A hydrogen gas content measuring apparatus used to practice such "Telegas" process, namely, a "Telegas" apparatus is a system which is adapted such that nitrogen or other inert gas is brought into contact with the body of molten metal by feeding the inert gas through one of two bores in an immersion head immersed in the molten metal while the inert gas in the molten metal is collected by the other bore and recirculated, through a pump, a hydrogen content detector, so-called katharometer, and the immersion head, back into the body of molten metal, this circulation of the inert gas being repeated until the pressure of the hydrogen gas picked up from the molten metal by the inert gas is in equilibrium with the content of hydrogen dissolved in the molten metal. Then, the mixture gas containing the hydrogen gas thus obtained through repeated circulation of the inert gas is measured in thermal conductivity by measuring electrical resistance variations of one hot-wire type detecting element (electric resistance wire) disposed in one of two measuring cells provided on the previously indicated hydrogen content detector. In the meantime, the other measuring cell or comparator cell is regularly charged with atmosphere whose thermal conductivity is substantially equal to that of the inert gas (nitrogen in this specific embodiment) flowing in said one measuring cell, whereby the electrical resistance of another hot-wire type detecting element is kept at a fixed value which is used as a reference with which the electrical resistance of said one detecting element is compared by an electric bridge circuit or network. More specifically, the difference between the two resistance values is used to obtain variations in thermal conductivity of the inert gas corresponding to the magnitude of partial pressure of the hydrogen in the circulated inert gas surrounding said one detecting element. In other words, the electric bridge circuit uses the electrical resistance differential value to obtain an out-of-balance current representing the magnitude of the equilibrium pressure of molecular hydrogen. This out-of-balance current is converted into a value of hydrogen gas content with reference to a calibration curve which represents a relationship between out-of-balance current values predetermined according to temperature of the molten metal, and hydrogen gas content values. The obtained value of hydrogen gas content is multiplied by a compensation constant selected depending upon specific metals in order to obtain a target value of the content of hydrogen dissolved in the molten metal.
Although the above conventional Telegas apparatus is capable of measuring directly the content of hydrogen (concentration of hydrogen gas) dissolved in a molten metal in a shorter period of time than with other methods wherein the concentration of hydrogen gas is measured by using a sample of solidified metals in question, such Telegas apparatus has not been considered completely satisfactory in operating efficiency for the reasons stated below.
The Telegas apparatus in the art uses a hydrogen content detector whose measuring cells are accommodated directly in bores formed in a housing block of brass or other similar metals. In such structure, a considerably long period of time is required before a thermal equilibrium has been reached among two hot-wire type detector elements of an electrical bridge network, metal block accommodating those elements, and a atmosphere surrounding the metal block. As a result, a comparatively long period of time is required before the reading of an ammeter connected to the bridge network has been stabilized so that the content of hydrogen in the molten metal is obtained.
The Telegas apparatus is also disadvantageous in that variations in temperature of the atmosphere surrounding the metal block will break the thermal equilibrium of the metal block with the hot-wire-type detector elements received within that block, thereby causing gradual variation in reading of the ammeter connected to the bridge network. This phenomenon not only requires an appreciable length of time to ensure that the ammeter reading has been restored to a steady value, but also requires re-calibration or zero-adjustment of the ammeter each time the thermal equilibrium is lost due to temperature variation of the atmosphere.